For an inkjet-type image-forming apparatus, a highly-accurate position control is generally required in driving paper-carrying in order to obtain a high-quality image. Here, as it is preferable to be able to directly observe the motion of a paper-carrying belt to be position controlled, it is possible to directly mark on the paper-carrying belt and detect the mark (see JP 2002-238274A). Moreover, a method of affixing a linear scale on the front or back face of the paper-carrying belt is known as a way of actually providing the mark.
FIG. 1 illustrates an image-forming apparatus in which a linear scale is provided on a paper-carrying belt. The illustrated image-forming apparatus 1 is set to be a mechanism such that a motor 3 driving via a decelerating mechanism a drive roller 4 installed causes a paper-carrying belt 2 provided between the drive roller 4 and a follower roller 5 to be rotated and paper (not illustrated) to be carried. In this carrying process, printing (image forming) is performed on the paper by use of an inkjet (IJ) carriage 6.
A linear scale 7 is provided on the inner side of the paper-carrying belt 2 over one round of the belt for measuring the belt position. Moreover, one sensor 8 for reading this linear scale 7 is provided at a position opposite the linear scale 7.
This sensor 8 is connected to a detection apparatus 102 in which the number of sinusoidal peaks (the number of pulses) generated in correspondence with the movement of the paper-carrying belt 2 is counted based on the sensor output sent from the sensor 8. The count value in correspondence with the movement of the paper-carrying belt 2 is sent to a control apparatus 103 which controls, based on the count value, a drive apparatus 104 for driving and controlling the motor 3. Hereby, the moving speed of the paper-carrying belt 2 is controlled by a predetermined speed, thus making it possible to form a high-quality image.
However, circumferentially affixing a scale onto the paper-carrying belt 2 inevitably results in the scale crossing a seam portion. Thus, at the seam portion, the scale interval (the phase) of the linear scale 7 is caused to be discontinuous and also the sensor output from the sensor 8 is caused to be a discontinuous signal.
In other words, even when the paper-carrying belt 2 is moving at a constant speed, the sensor output from the sensor 8 is caused to be discontinuous, resulting in not being able to obtain correct position information. The stopping of the flow of information necessary for position control in the seam portion results in a problem of not being able to form highly-accurate images on paper.